Think through all of the skills that you anticipate learning in the position. Since you won't be paid, experience is what you're getting out of the internship. Try to quantify what you'll learn from the internship; for instance, if you'll learn specific skills like Excel or programming or how to use a certain software, or more general skills like the ability to communicate. One big benefit of any internship is that, if you do well, you can ask your supervisor for a recommendation in the future. If you think that you'd learn a lot and excel in the internship, it's probably worthwhile in that it will help you a lot when you're looking for future jobs.

Sometimes money is not the most valuable thing you can receive in college. An unpaid internship should being offering you experience in the field and opportunities to network with people in the field that could potentially lead to a paid internship or job after graduation. You should be gaining experience that you would not get without the internship to make it worth your time.

Unfortunately, unpaid internships are extremely common, and paid internships are a bit of a rarity. That being said, unpaid internships are still worth your time.

First of all, you'll want to consider the workload: is this too much work to be doing for no pay? Can you handle this amount of work?
Second, you'll want to consider the time commitment: can you balance the internship hours with your coursework without sacrificing quality?
Third, you'll want to consider the experience: will this position provide you experience that will look good on your resume? Is the internship applicable to your desired career path?

Unpaid internships shouldn't be thrown out the second you see the term "unpaid" - sometimes, that unpaid internship is going to be the difference between getting a position and not. Internships are a great way to gain experience, paid or not. So definitely consider them just as carefully if they're unpaid.